This invention pertains to an apparatus and a method of producing ultra-thin walled extruded polymer products using a polymer extruder. Polymer extruders are used to produce polymer tubes and ducts and to coat with polymers circular, rectangular, stranded and coiled conductors, such as electrical wires, ribbons, cables and coils.
A common type of extruder employed in manufacturing such extruded polymer products is a xe2x80x9cxe2x85x9c-inch single screw cross-headxe2x80x9d extruder 300. In such an extruder, polymers in the form of pellets are placed in a feed hopper 310 and thus fed into an extruder barrel 320. See FIG. 10. Extruder barrel 320 houses a helical extruder screw 330. It should be noted that commercially available pellets must be repelletized, i.e., resized, to a smaller size for use in xe2x85x9c inch extruders to avoid damage to the extruder screw. The polymer fills the spaces between the surface of extruder screw 330 and the interior walls of extruder barrel 320. The screw is rotated about its longitudinal axis by an electric motor 340 while extruder barrel 320 remains stationary. The rotation of extruder screw 330 transports the polymer through extruder barrel 320 creating pressure and friction between the polymer and the interior walls of the extruder barrel 320. The combination of pressure, friction and additional heat provided by heaters melt the polymer. In polymer extrusion, the additional heat is most commonly supplied by electric resistance heaters, which are placed along the exterior of extruder barrel 320.
By the time the polymer has traveled the length of the extruder barrel, it is completely melted. The molten polymer, i.e., polymer melt, is then forced through a breaker plate 345, which is housed in the body of the adapter 346. Breaker plate 345 causes the polymer melt to flow in a linear direction as opposed to a helical direction.
Breaker plate 345 is a metal cylinder which provides five channels, for polymer melt flow, running along the length of the cylinder. For example, the breaker plate that is provided in a typical xe2x85x9c-inch extruder is approximately 0.377 inches in length and has an overall diameter of approximately 0.748 inches and provides five channels each having a diameter of approximately 0.110 inches. Accordingly, the overall cross-sectional area of the standard breaker plate is 0.439 square inches and the cross-sectional area provided for polymer flow is approximately 0.047 square inches (the sum of the cross-sectional area of all five channels). Accordingly, the ratio of the total cross-sectional area provided for polymer flow to the overall cross-sectional area of the breaker plate is 0.107.
Breaker plate 345 may also support a filter which is used to remove contaminants from the polymer melt. Typical filters used in polymer extrusion range from 100 to 400 mesh (100-400 lines per square inch).
The polymer melt, after flowing through the breaker plate and filter exits the adapter and enters a crosshead assembly 350 where it is forced through an extruder die 360. The polymer melt emerging from the extruder die 360 is referred to as an extrudate. The shape of the extrudate immediately leaving the extruder die is not the final shape. For example, in wire coating, a wire 318 travels along a wire path through the crosshead assembly where it comes into contact with the polymer melt which coats the wire. Upon emerging from extruder die 360, the walls of the polymer coating rather than being uniformly concentric and parallel forms a cone around the wire. This phenomena is partially attributed to extrudate swell. As the wire is further drawn away from the extruder die, the coating walls become uniformly parallel.
Currently available extruders are unable to effectively produce ultra-thin wall, less than 50.8 microns (0.002 inch) in wall thickness, pin-hole free, polymer products. This inability is in part due to the presence of polymer melt contaminants, such as gels and thermally degraded polymers, and the Theological properties of the polymer. Ultra-thin coating is necessary in biomedical implants, where wires with diameters as small as 25.4 microns (0.001 inch) are used and must substantially retain their inherent flexibility and small diameters. Complete coverage of the wire with polymer is necessary to prevent unintended contact between the bare conductor and body fluids and tissue. When attempts to place ultra-thin coatings on such wires have been made, the resulting coating is incomplete or covered with pinholes.
In addition, currently available extruders do not provide an effective method for instantaneous visual inspection of the ultra-thin extrudate. Such inspection would be advantageous as it would allow an extruder operator to determine whether the extrudate is being uniformly formed, i.e., that the polymer coating extruded on a wire is uniform in thickness and concentric. Consequently, an extruder may be operated for a long period before any defect is noticed. This results in wasted material and loss of production time.
Non-uniformity of the extrudate walls may be corrected by adjusting the position of the extruder die 360 along different lateral axes. However, such extruder die adjustments are made cumbersome by the current adjustments mechanisms incorporated in currently available extruders (see FIG. 11). Present extruders commonly employ four adjustment screws 370 that act directly on die 360 to adjust the die""s position. Consequently, adjusting the die position is time consuming because each screw must be manipulated to adjust the die. On many small extruders, such as a xe2x85x9c-inch extruder, at least one of the four adjustment screws 370 is placed in a difficult to accessed location. Unlike thicker walled wire coating, attempts to produce thin walled polymer coatings over wire do not provide the capability to adjust the concentricity of the coating without stopping the coating process. Consequently, the extruder must be stopped to make time consuming die adjustments. This results in numerous trial and error runs to achieve a uniform product.
In one preferred aspect, the present invention is an extruder with a cross head assembly that comprises a heat source to heat the extrudate as it emerges from the extruder die. It has been discovered that subjecting the extrudate emerging from the extruder die to a short period of heat allows for ultra-thin walled extrudates to be produced without shearing or the formation of pin-holes. Furthermore, the application of heat to the emerging extrudate provides additional flexibility in regards to residence time and temperatures in the extruder barrel and the cross head assembly. Accordingly, because of this additional flexibility, polymer thermal degradation may be minimized, thereby reducing the formation of polymer gels and other polymer melt contaminants.
In a second preferred aspect, the present invention is an extruder that provides instantaneous inspection of the extrudate. This second aspect incorporates the use of video equipment, to provide magnified observation of the extrudate, and associated mirrors to observe the extrudate as it exits the die along multiple views.
In a third preferred aspect, the present invention is an extruder with a cross head assembly that provides a die holder and an extruder die which allows more expedient die adjustments to adjust the shape of the extrudate without stopping the wire coating process.
In a fourth preferred aspect, the present invention is an extruder with an improved breaker plate to enhance filtration, reduce extrudate residence time thereby reducing polymer degradation and formation of contaminants.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.